For the application of paints, adhesives or other coating materials to shaped parts made of plastic, the wettability thereof is of decisive importance. The organic plastics frequently used in practice, such as polyethylene (PE), polypropylene (PP), polyurethane (PU), acrylate/butadiene/styrene copolymers (ABS), polybutylene terephthalate (PBT) and polyamide (PA), as well as blends thereof have low surface energies and therefore have only very limited wettability. In order to be able to paint, coat or otherwise treat such plastics, a pretreatment of the surface of the plastic is absolutely essential.
In known methods, the shaped parts made of plastic are treated in particular by treatment with fluorine-containing gases. As a result of the fluorination, the originally substantially non-polar surface becomes very much more polar. At the same time, the surface is “chemically roughened” so that coating materials, such as paints, are better mechanically anchored thereon. As a result of increasing the polarity and the roughening, a mechanically strong bond stable for a long time is achieved between the part made of plastic and the coating medium present thereon (DE 101 08 723 A1; DE 198 32 559 A1; EP 0 730 948 A1).
Methods for fluorination in the presence of oxygen are also known. Thus, EP 0 629 654 B1 discloses a method for the pretreatment of those surfaces of parts made of plastic which are to be coated, in which method the parts made of plastic are treated with a gas mixture which comprises 3 to 5% by volume of fluorine, 10 to 16% by volume of oxygen and additionally at least one further inert gas, in particular nitrogen. The treatment is carried out in general so that the shaped parts are introduced into a reaction chamber and the air pressure in the reaction chamber is reduced to 200 mbar. A mixture of fluorine and nitrogen is then passed into this residual air. The amount of the gas mixture passed in is chosen so that the pressure in the reaction chamber increases by 100 to 200 mbar per minute until a total pressure of 333 to 400 mbar is reached. The fluorine-containing gas acts for about 10 to 180 seconds altogether on the shaped parts made of plastic. The method is carried out at reduced pressure; appropriately strong reaction chambers and vacuum pumps are therefore required. In addition the method requires a relatively long time. Moreover, a relatively high proportion of fluorine is employed. Fluorine from the treatment gas is incorporated into the surface of the shaped part in a measurable amount.
DE 38 40 269 C2 discloses a method for producing smooth surfaces on articles made of polymers of ethylene, propylene, butadiene or polystyrene. For this purpose, the articles are exposed to a gas mixture comprising 0.01 to 0.5% by volume of fluorine, 0.5 to 21% by volume of oxygen and nitrogen. In the examples of the DE-C, a gas mixture in which the volume ratio of fluorine to oxygen is 1:6.5 is used. The fluorinated surface layers produced therewith comprise substantially measurable amounts of fluorine (about 6 μg of fluorine per cm2) and—in contrast to surfaces fluorinated in the absence of oxygen—are distinguished by a uniformly smooth structure. Mechanical interlocking with a paint or adhesive layer applied to said surface therefore cannot be achieved. Accordingly, a strong bond does not exist between the different materials. An increase in the polarity of the surface of the shaped parts with simultaneous chemical roughening was achieved to date only by fluorination in the absence of oxygen. The atmospheric oxygen had to be removed from the reaction chamber before the fluorination by flushing with inert gas.
It was therefore the object to provide a method by means of which the polarity of the surface of shaped parts made of plastic can be increased and by means of which at the same time (chemical) roughening can be carried out. The method should be easy to carry out—i.e. even at atmospheric pressure. It should manage without time-consuming flushing with inert gas. The shaped parts should be particularly readily coatable after the treatment and the paint coat should have a strong durable bond to the shaped part.